Production of gas from subterranean oil and gas reservoirs by drilling and installation of grouted casings is a well-established practice. Natural gas (methane) production has primarily been achieved through drilling wells into deep reservoirs where natural gas, frequently in association with crude oil and water, may be trapped under a layer of cap rock. The well is lined with a casing that is cemented to the surrounding formation to provide a stable wellbore. The casing is then perforated at the reservoir level to allow gas and reservoir fluids to flow into the casing and then to the surface through tubing inside the casing.
In these cased well applications, one or more concentric casings are installed to progressively greater depths, down to a pressurized reservoir. Cementing, or grouting, the casing(s) to the formation material, and to adjacent casings, prevents hydrocarbons from escaping from the pressurized reservoir along the exterior of the casing. Gas enters the lower part of the casing via perforations in the casing or, in highly consolidated (rock) reservoir formation material, via an un-cased extension of the drilled hole.
In most applications, a “packer” is used to isolate the lower part of the casing from the upper part and one or more strings of production tubing hang from the wellhead down to the zone below the packer or between adjacent packers. After entering the casing via the perforations, the gas enters the tubing string(s) where it flows to the surface, through valves, and to a pipeline. The cased well method facilitates control of the flow of gas from a high-pressure reservoir and is well suited for production from porous rock or sand formation material.
Methane hydrates, or hydrates, are one type of formation material found close to the surface, especially in cold environments. Methane hydrates are similar to water ice and are composed primarily of water, methane, and, to a lesser extent, other volatile hydrocarbons. The frozen water particles form an expanded lattice structure that traps the methane, or other hydrocarbon particles, to form a primarily solid material.
Methane hydrates have been found to be stable over a range of high pressure and low temperature. Methane hydrates are stable at combinations of temperature and pressure found in onshore arctic regions and beneath the sea floor in water depths greater than approximately 1,500 feet (500 meters). Changes in either the temperature or the pressure can cause methane hydrates to melt and release natural gas. Methane gas may also be trapped below the hydrate layer, much as it is trapped below cap rock layers in deep underground reservoirs.
The development of viable methods for the commercial production of natural gas from naturally occurring deposits of methane hydrates has been the subject of extensive research. The construction of standard cased wells has been used to reduce the pressure on the underside of the hydrate-bearing zone. This approach collects gas that is trapped below the hydrates and, by reducing the pressure, may cause hydrates in the surrounding formation to release additional natural gas. This release will cease when the formation materials isolate the remaining hydrates from the zone of reduced pressure or when the latent heat of thawing causes the temperature to drop sufficiently to stabilize the remaining hydrates at the reduced pressure. Thawing absorbs heat equal to the latent heat of the hydrates and, if this heat is not replaced, the temperature will drop and conditions will eventually shift into the stability region for hydrates, whereupon release of methane from the hydrates will stop.
Notwithstanding the above teachings, there remains a need to develop new and improved methods and apparatus, for producing hydrocarbon gases from subterranean hydrates, which overcome some of the foregoing difficulties while providing more advantageous overall results.